Powder metallurgy



Patented Jan. 2, 1945 POWDER METALLURGY Nils K. G. Tholand, Bethlehem, Conn.

No Drawing. Application October 10, 1940, Serial No. 360,664. In Sweden October 14, 1938 Claims.

An object of the present invention is to provide for the inexpensive production of shaped articles or objects such as gears, bushings, small automotive parts, hardware articles, adding and typewriter machin parts, magnet cores and the like, and rods, bars, wire, sheets, billets, -etc., capable of being worked by machining, forging, etc., from powdered or finely divided ferrous metal and alloys by powder metallurgical methods involving pressing and sintering.

It has been customary heretofore in the powder metallurgy of iron to employ iron powder produced by electro-deposition, iron powder produced by hydrogen reduction of iron oxide, carbonyl iron or sponge iron powder. These sources of the iron powder are, however, relatively expensive and the roducts do not have the desired composition and physical properties for many purposes.

Many attempts have been made to produce articles by powder metallurgical methods having the composition and physical properties of steel and of ferrous metal alloys.

It has now been found that powdered metal suitable for powder metallurgy may be made by procedures involving the subdivision of carbon containing ferrous metal and decarburization of the subdivided metal as described in Reissue Patent No. 21,500. Said patent discloses the decarburization of ferrous metals, such as pig or cast iron, of high carbon content which may contain alloying elements, such as Si, Cr, Mn, W, V, Mo, Ni, Co, tc.. According to said patent, the metal may be subdivided by mechanical crushing and grinding or by subdividing the metal in liquid state to prepare it for the decarburizing treatment. The process offsaid patent is, however, designed to produc a metal suitable for use as a substitute for scrap iron when making steel in open hearth and electric furnaces and has not been operated in such a way as to roduce a metal powder suitable for powder metallurgy. As a matter of fact, the process of the patent generally employs the subdivision of the metal in the liquid state to a particle size of 1-10' mm. diameter because rounded grains within this size range are most convenient for the decarburizing operation and the product is most satisfactory for the intended purpose.

A metal powder to'be suitable for powder metallurgy should have a particle size not greater than about 40 to 60 mesh and we have found that the particles should have the shape produced by mechanical subdivision of the metal in the solid s a i state and that the production of a metal powder of this particle size and shape is not inconsistent with the decarburization 0f the metal in accordance with the process of the patent.

On the contrary, the high carbon content of the ferrous metal which gives rise to th necessity for decarburization is a contributing factor in the subdivision of the metal to the size and shape necessary for powder metallurgy and, on the other hand, the subdivision of the metal to th size and shape, necessary for powder metallurgy serves for the decarburization. Further, it has been discovered that the finely divided decarburized metal possesses excellent properties for powder metallurgy. Besides its low cost and completely controllable composition, it has excellent-pressing properties which are attributed to microscopic porosity probably resulting from the removal of graphitic carbon from the metal without fusion.

. In accordance with the present invention, therefore, the high carbon ferrous metal of suitable composition is subdivided to a particle size of say 40 to mesh or finer by any suitable procedure involving at least a final mechanical disintegration of the metal in the solid state and is then decarburized as described in the patent.

A variety of steps may be associated with this basic process of mechanical disintegration and decarburization of the high carbon ferrous metal in the solid state. The process starts with a molten metal of controllable composition, such as pig iron containing the usual amounts of silicon and manganese, e. g., from a trace to 1% or more of silicon and from 0.1 to 2% of manganese, and a minimum of about 1.5% of carbon, although the carbon content of the ferrous alloy may be higher. The metal is subdivided in the liquid state and solidified to facilitate the mechanical disintegration thereof in the solid state. Preferably the metal. is subdivided in the liquid state to granules or shot of say 10 to 20 mm. diameter or less, solidified and then crushed and ground to a powder. The metal should contain suiflcient carbon that when it is removed by the decarburizing process the desirable porosity is produced.

It appears that the porosity depends upon the removal of graphitic carbon which, depending upon the previous treatment of the metal, may be present in the metal at the time of the mechanical disintegration. Graphitic carbon probably is not present in the metal produced by subdivision of the metal in the liquid state to the form of granules and mechanical disintegration of the granules but may be precipitated either by a separate heat treatment of the granules or powder or by the heating which accompanies the decarburization treatment. Due to the fact that the metal is subdivided in the liquid state and the resulting relatively fine particles very quickly cooled, the rules concerning the conditions necessary for the formation of graphitic carbon, 1. e., a carbon content 01' at least 1.7% and a high silicon content, which rules have been determined with reference to massive castings, probably must be applied with some modification. Applicants do not, therefore, wish to rely upon any particular theory as to how or why the porosity of their metal powder occurs but rather to rely upon the fact that when a ferrous metal of high carbon content (at least 1.5% in the case of unalloyed pig iron) is subdivided in the liquid state, mechanically disintegrated in the solid state and decarburized in the solid state, with or without heat treatment designed to precipitate graphitic carbon, it does have a microscope porosity which contributes to the utility or the resulting powder in powder metallurgy.

The decarburization is carried out as disclosed in said Reissue Patent No. 21.500 to the desired carbon content in the finished metal powder, say 0.05 to 1.25%. The decarburization should be so conducted by control of the decarburizing gas composition and volume and the cooling of the powder in an inert or reducing gas that the metal particles are sufliciently free of ox de film for powder metallurgical purposes, or the decarburized powder may be subjected to treatment in a se arate step to remove oxide film, e. g., by heating the powder in a reducing gas such as hydrogen.

It may be desirable to classify the metal particles at any one or more 01' several po nts in the process, e. g., after the liquid subdivision of the metal and before the mechanical disintegration of the solid metal or after themechanical disintegration and before the decarburization or after the decarburization. The purpose and utility of classification are well understood but it may be observed that classification may be useful or even necessary in connection with certain methods of mechanical disintegration and also useful in connection with the decarburization and further useful in mak ng a powder of to heat treatment to convert its carbon content.

to graphitic carbon or to develop the graphitic structure. The decarburized metal may be further mechanically subdivided with or without classification and/or quenching.

As stated, the metal powder must be of a suitable particle size for powder metallurgy, say at least 40 to 60 mesh and preferably 100 mesh or finer. It must have a particle shape produced by mechanical disintegration of the metal in the solid state. It must be sumciently free of oxide film. It must have the desired composition with respect to carbon content and alloying'ingredients and itshould have the porosity produced by the removal of carbon from the metal by decarburization in the solid state.

The specific procedure to be followed will de pend upon the circumstances in each instance,

etc.

such as thecomposition of the metal, the method of subdividing the metal in the liquid state to be employed or the form of the metal following its subdivision in the liquid state, the method of mechanical subdivision of the metal in the solid state, the particle size and composition of the powder desired, etc. It will be appreciated that these factors may be correlated to each other and to other operations such as quenching, mechanical disintegration after decarburization, treatment to remove oxide film, classification, etc., in a great variety of combinations of which the mechanical disintegration and decarburization of the metal of high carbon content constitute the nucleus.

Any of the various known methods of powder metallurgy may be applied to the metal powder. It may, for instance, be compressed in admixture with volatile materials such as stearic acid, ammonium chloride or the like and then subsequently heat treated or sintered so as to produce a porous article capable of being impregnated with lubricant. The powder maybe simply compressed in the cold and later subjected to heat treatment, or it may be hot. pressed, 1. e., simultaneously subjected to heat and pressure and it may be later subjected to further consolidating heat treatment or not, as the case requires. These operations have as their object the welding or sintering of the particles together. In the case of cold pressing, the heat treatments can be conducted at temperatures of from say 550 to 1300" C. Hot pressing is conducted at slightly lower temperatures but in both cases the sintering temperature selected depends upon the composition of the powder and the pressures employed. The powder may be pressed in such a way as to produce articles which are substantially free of porosity and the density of which approaches that of the same metal composition in cast form. Such high density articles may of course be made by either hot or cold pressing and with a sintering after treatment of the pressed article. The articles may be machined or mechanically worked, e. g, forged, rolled, drawn, The high density non-porous products are preferred where high strength is required. The metal powder may also be mixed with an insulating material such as a varnish 0r resin for insulating the particles in the production, for instance, of cores for electrical coils.

The ferrous metal powder may, if desired, be mixed with a small proportion of a low-melting metallic constituent such as copper which, when the pressed article is heated, melts and serves to cement the ferrous metal particles together and even to form a more or less continuous matrix for the ferrous metal particles, filling the voids between them. Further, the ferrous metal powder may be mixed with abrasives, such as carborundum, crystalline alumina, vitrified zirconla, or hard metallic carbides such as tungsten carbide, titanium carbide, tantalum carbide or the like. Also the metal powder may be mixed with solid lubricants such as graphite. Such abrasives and solid lubricants are thus incorporated into the finished article. The metal powder may also be mixed with constituents intended to be incorporated into the metal particles themselves and thus modify their composition and properties. For example, the ferrous metal powder before being pressed may be mixed with silicon, calcium, manganese or other alloying metal in powdered form. Such added materials are thus incorpo rated into the pressed ferrous metal articles in the metal powder of the present invention is adapted for use in powder metallurgy procwses.

a manner which is not usual in cast articles of the same composition nor in similar articles made from steel or alloy powders of the same composition. Grinding the ferrous metal with the added material and/or the sintering of the pressed articles, either or both tend to improve the incorporation of the added material.

The metal powder produced directly by the decarburizing treatment or after further mechanical subdivision may, as stated, be subjected to the action of a reducing gas such as hydrogen, carbon monoxide or illuminating gas to remove any oxide film from the powder and the powder may be protected against oxidation during the grinding operation and/or during the pressing operation by means of a reducing or inert gas.

The formation of metal articles such as mechanical and electrical elements by powder. me-

tallury' is a well known procedure and need not be described in detail excepting to state that generally which involve pressing and sinterlng of the powdered metal. Normal pressures may be employed and the powder, due to the shape of the particles produced by the mechanical subdivision of the high carbon brittle metal and the porosity produced by removal of carbon, exhibits excellent cohesiveness and compressibility. The sintering temperature to be employed will depend upon various factors well known in the I art such as the composition of the metal.

It will be appreciated that an outstanding advantage of the present invention is that it produces directly by a very simple and economical process a metal powder having physical characteristics fitting it"for powder metallury and of any desired and readily controllable composition. Thus, for instance, steel articles may be produced directly from cast or pig iron of homogeneous composition without the necessity of subdividing steel as such and without converting soft iron into steel by incorporating-alloying elements in the powder metallury process, both of which are expensive and unsatisfactory operations.

, This application is a continuation-mm of application Serial No. 298,334, filed October 6, 1939.

I claim:

l. The method of making powdered metal I ing ferrous metal in the liquid state, solidifying the subdivided metal, mechanically disintegrating the solidified metal to a particle size not substantially, greater than mesh, and decorburizing thesubdivided metal in-the solid state under such conditions as to remove graphitic carbon.

3. The method of making powdered ferrous metal which is suitable for powder metallurgy which comprises mechanically disintegrating and decarburizing in the solid state under such conditions as to remove graphitic carbon a carboncontaining ferrous metal to a particle size not substantially greater than 40 mesh and shape suitable for powder metallurgy.

4. Metal powder of a particle size not substantially greater than 40 mesh produced by mechanical disintegration and decarburization in the solid state under such conditions as to remove graphitic carbon of a carbon-containing ferrous metal, the particles of said metal powder containing voids.

5. Metal powder of a particle size not substantially greater than 40 mesh produced by mechanical disintegration and decarburizatiomin the solid state under such conditions as to remove graphitic carbon of a carbon-containing ferrous metal, the particles of said metal powder containing inclusions of flux.

NILS K. a. mom. 

